Known injection molding machines having a reciprocating screw injection system and a straight hydraulic mold clamping system are of complex, expensive construction. For example, known straight hydraulic clamping systems generally comprise a mold clamping cylinder having its piston secured to the movable mold supporting platen, said cylinder having a pullback area to which fluid under pressure is supplied to move the piston in mold opening direction and having a central blind bore slidably fitted on a fixed tubular booster or jack ram, said last-mentioned ram having a spring and pressure actuated prefill valve slidable thereon which opens communication between a prefill tank and the chamber of the clamping cylinder during the opening and closing strokes of the mold clamping piston and which is actuated by fluid under pressure brought in through the tubular booster ram to close communication between said chamber and said prefill tank and to open communication between the fluid pressure source and said chamber to pressurize the trapped fluid therein so as to exert required mold locking force for injection of plasticized material into the closed mold cavity.